1. Field of the Invention
The present invention relates generally to a cathode ray tube apparatus, and is directed more particularly to a cathode ray tube apparatus in which when a discharge is caused in a cathode ray tube, the discharging current is suppressed so as to induce a voltage by the discharge only in the tube thereby to reduce undesirable influences which are exerted on the cathode and heater of the cathode ray tube and the electrodes of an electron gun and so on as much as possible and hence to reduce undesirable influences on the circuit elements forming the cathode ray tube apparatus.
2. Description of the Prior Art
In the prior art cathode ray tube apparatus, discharge is frequently generated in the tube or envelope. For example, in a color cathode ray tube apparatus of the Trinitron (registered Trade Mark) system, as shown in FIG. 1, for a plurality of cathodes Kr, Kg and Kb in an envelope 1, arranged are common first to fifth grids G1 to G5 on the same axis, and a convergence device CD, which is formed of four electrodes and disposed at the front end of fifth grid G5 to form an electron gun 2. In this case, an anode voltage is applied to the fifth and third grids G5 and G3, which are electrically connected through a high voltage lead wire 6, from the outside of the envelope 1 through, for example, an anode button (not shown), inner carbon layer 3 and conductive contact spring 4, while a predetermined low voltage is applied to the first, second and fourth grids G1, G2 and G4 through a stem pin 5. Though not shown, the inner deflection plate of convergence device CD is applied with the anode voltage through the contact spring 4 and the outer deflection plate of convergence device CD is applied with a middle high voltage, which is somewhat lower than the anode voltage, from the coaxial anode button through, a high voltage supply pipe (not shown).
With the above prior art color cathode ray tube apparatus, there may occur a case where a discharge is generated between the high voltage electrodes or fifth and third grids G5, G3 and the lower voltage electrodes or fourth and second grids G4, G2, in detail between the fifth grid G5 and fourth grid G4 or between third grid G3 and fourth grid G4, or between the third grid G3 and second grid G2, respectively. The above discharge is the discharge of the electrical charges stored in a capacitor formed between the inner carbon layer 3 and the outer carbon layer 7. In this case, a current i flows into the outer carbon 7 through the path indicated by the arrow in FIG. 1.
FIG. 2 is an equivalent circuit of the above discharging current path. In FIG. 2, 8 designates the discharge gap, R a total resistance, C the above defined capacitance and L a total inductance, respectively.
When the discharge occurs, it can be ascertained by measurements of a current peak of several hundred amperes which is reached in 100 to 200 nanoseconds and the time rate of current change reaches about 10,000 A/.mu.s. It is obvious that such large currents seriously affect the cathode ray tube and the circuit elements disposed near the cathode ray tube.
In order to reduce damage caused by the above discussed discharge, it is sufficient to introduce a high impedance within the discharge path. To this end, in the art such a system has been known, in which resistive particles are mixed into the inner carbon layer 3 to give a predetermined resistance value thereto to reduce the discharge current. This is known as a so-called resistive carbon coating method. In another method a resistive layer is coated on the contact spring 4, contacting with the inner carbon layer 3 and applying the high voltage to the electron gun 2, to reduce the discharge current, which is known as the resistive contact-spring system.
In any of the prior art systems, as will be clear from the above description, the resistive material serving to reduce the discharge current is provided within the cathode ray tube for the following reasons. That is, in order to remove the undesirable influences by the discharge on the respective parts of the cathode ray tube and the circuit elements thereabout, the following three points are important.
(1) The discharge current is suppressed. PA0 (2) The rising-up of the discharge current is to be low i.e. the differentiated value of the discharge current with respect to time di/dt is made small, and PA0 (3) The induction of high voltage by the discharge is generated within the cathode ray tube.
It is not necessary to explain the above point (1). As to point (2), since a ground lead wire 9, which is connected between the side of stem pin 5 and the outer carbon layer 7, has a small coil or inductance component to induce a high voltage at the side of stem pin 5 in accordance with the equation E=L.times.di/dt upon discharging and the magnetic material near it is magnetized by the rising-up factor di/dt or voltage is induced in the deflection yoke to damage the circuit elements, the rising-up factor di/dt must be made small. In order to satisfy the points (1) and (2), it may be desirable to introduce an impedance to the outside of the cathode ray tube, for example, the ground lead wire 9. However, if the impedance is provided at the outside of the cathode ray tube as set forth above, upon discharging a high voltage is induced at the side of the stem pin 5 to undesirably affect the circuit near the same. For this reason, when the impedance for preventing the discharge current is provided, it is selected to be only within the cathode ray tube in the prior art.
However, in the prior art resistive carbon coating system or resistive contact-spring system, are accompanied with the following defects.
First, the resistive carbon coating system will be described. In the resistive carbon coating system, as shown in FIG. 3, the resistance value of the inner carbon layer 3 in contact with the conductive contact spring 4 is selected to be high. In this case, however, the contact between the contact spring 4 and the carbon layer 3 results in the peeling-off of carbon layer 3 to increase the possibility of discharge. Even if the discharge is caused frequently, the discharge is feeble (soft). Therefore, undesirable influences on the circuit neat it will be caused, but the picture flickers frequently during discharging which is uncomfortable for a viewer. Further, due to the fact that the contact of the spring 4 with the carbon layer 3 is a point contact, the deterioration of the carbon layer 3 proposes a problem. That is, during a discharging or nocking treatment, a current will flow through a very small portion, which may deteriorate the carbon layer 3 due to the heat generation and hence lower the reliability. In addition, when a gas absorbing agent called a getter which is highly conductive adheres to the carbon layer 3, the effect of carbon layer 3 and its resistance disappears. Therefore, it is necessary to shield an area A of the carbon layer 3, which may make contact with the contact spring 4, so as to prevent the adhesion of at least the getter to the carbon layer 3. In fact, however, it is difficult to positively shield the area A of the carbon layer 3. In this case, a device to shield the area A is required, which results in the construction of the cathode ray tube becoming complicated.
To avoid a problem, such a method may be considered where a non-doped getter is used. This non-doped getter has a property such that the scattering of barium Ba by nitrogen gas N.sub.2 is not caused but barium is deposited directly, so that the shield can be easily constructed. According to the experiments, however, it was ascertained that the non-doped getter deteriorated the uniformity of the color on the picture screen and the area deposited with barium becomes small due to the non-porosity of the deposited barium layer, which resulted in reduction of the gas absorption effect and consequently in a short life span of the cathode ray tube. Further, even if the shield can be provided, the shield effect fluctuates, which results in the scattering of the discharge current and accordingly positive shielding effect is not assured. Also, the getter is generally attached to the electron gun 2 with a metal plate. In the resistive carbon coating system, however, if the electron gun and the getter are not isolated electrically, the discharge current will flow through the metal plate. To avoid this, such a technique is necessary where the getter is attached to a color selecting mask, anode button or the like, which results in difficulties during manufacturing process.
Further, the maintenance or control of the static resistance and the dynamic resistance is difficult. The static resistance means the quotient of several volts difference set between the carbon layers by the current flowing to the carbon layers by the voltage difference, and the dynamic resistance means the quotient of the high anode voltage divided by the peak value of the discharge current. Upon comparing the static resistance with the dynamic resistance, the dynamic resistance has a tendency to decrease. This difference is determined mainly by the surface condition and the inner construction of the carbon layer 3, because the discharge along the surface of the carbon layer 3 (surface discharge) is determined by its surface condition and the impedance (the resistance to AC) is determined by the inner construction of the carbon layer 3. The maintenance or control thereof becomes important to maintain the results certain. The control of the grain size of the carbon powders or the coating method thereof may be considered, but the resistive carbon coating system lacks the ability to prevent fluctuations of the shield effect by the getter.
Secondly, the resistive contact spring system will be now described. According to this system as shown in FIG. 4, a resistive layer 10 is coated on the surface of the contact spring 4, which contacts with inner carbon layer 3, to reduce the discharge. This system, however, is not practical. That is, even if the resistive layer 10 is coated on the surface of the spring 4 or disposed between the spring 4 and carbon layer 3, the thickness of the resistive layer 10 is small so that discharge along the surface will be caused. In other words, the voltage across the resistive layer 10 becomes a high voltage due to the discharge which will cause the secondary discharge along the surface due to the short length, so that the discharge can not be suppressed. Further, it is necessary to shield the resistive layer 10 so as to prevent it from being coated by the back flash of the conductive getter material. In addition to the surface condition and the inner construction of the resistive layer 10, there exists a problem due to the capacitance since, due to the short distance between the inner carbon layer 3 and the contact spring 4, a capacitance C1 exists therebetween. The equivalent circuit thereof can be as shown in FIG. 5. Thus, the high frequency current such as the discharge current flows through the capacitance C1. As a result, the impedance (AC resistance) thereof becomes low, and accordingly the effect by the provision of the resistive layer 10 is limited even if the static resistance is selected to be relatively high.